The present invention relates generally to the field of laser illumination. It relates more specifically to using laser light to mark locations, such as may be practiced in connection with machine vision systems.
Laser light markers are used in a wide variety of industrial applications. Such applications include: imaging deblurring, pattern recognition, matched filter imaging processing, half-tone dot or line removal, active optical feedback image processing, optical holography and exposure of photolithographic patterns in semiconductor device manufacture, optical alignment and three dimensional sensing.
For instance, U.S. patent application Ser. No. 505,722, filed on Apr. 5, 1990, in the name of Steven J. Gordon, entitled REAL TIME THREE DIMENSIONAL SENSING SYSTEM and co-assigned with the present invention, and the corresponding published PCT application PCT/US91/02055, published on Oct. 17, 1991, describes a machine vision apparatus that uses a projected laser pattern. The '722 patent application and the PCT publication are incorporated fully herein by reference, illustrated generally in FIGS. 1 and 2. Two cameras (a nominally left camera and right camera) receive and record visual information with respect to a three dimensional object of which spatial information is desired. The object is illuminated by one or more coherent monochromatic laser light sources, which generate a plurality of simultaneous, spaced planes of light. The number of planes depends on the desired resolution, size of the object, and computational capacity of the data processing equipment. Each camera simultaneously records a still video image of the light reflected from the sensed object during the sampling period of the camera image capture device. Each image recorded is the pattern of light created by the reflection of the incident light planes off of the sensed object as seen from the position of each respective camera. The pixels of the video picture are converted into digital signal information and stored in computer memory.
After the video images are acquired and converted to digital signal information, each of the two images is processed, independently at first. First, for each image separately, signal processing is applied to the digital information to collect individual pixels into groups, each group comprising contiguous pixels in the image (hereinafter stripes). Signals representing the two video images are now combined to determine the coordinate location in three dimensional space of any illuminated point of the sensed object.
First, a single pixel of each stripe in one of the video images (e.g. the right) is selected (hereinafter the selected pixel). Given that the position of the camera lens in space is known and the selected pixel is known, then it is known that the point corresponding to the selected pixel (hereinafter the selected point) lies on a known line drawn from the lens center out into space. This line (which appears as a point in the right image), appears as a line in the left image. This line is called the "epi-polar" line of the selected point in the right image. Since the position of the lens center of the left camera also is known, this epi-polar line can be calculated and drawn in the left image. The epi-polar line, when drawn in the left image, will intersect at least one, and most likely several, of the stripes of the left video image. It is known that one of the pixels where the epi-polar line and a stripe intersect (hereinafter intersection pixels) represents the selected point from the right image. The actual coordinate location in space of the point corresponding to any of these intersection pixels (hereinafter intersection points) is determinable by triangulation. Since the position in space of each plane of light which created the stripes is also known, the single point of all the intersection points which corresponds to the selected point in the right image is ascertained by determining the three dimensional coordinate of each intersection point to determine if it lies in one of the known planes of light. The intersection point which lies closest to a known plane of light is taken as the selected point.
This process is repeated for at least one point in every stripe in the right image. Every stripe in the right image does not necessarily have a matching stripe in the left image. It is possible that stripes that appear in the right image, particularly stripes near the edges, will not appear in the left image. Most, if not all, stripes in the right image are matched to stripes in the left image by this process and the three dimensional coordinates of at least one point of each stripe is determined. With this knowledge and the knowledge that every other point in any given stripe lies in the corresponding light plane, the three dimensional coordinates of any other point in the group can be determined by triangulation. Thus, the coordinate location of any illuminated point on the sensed object can be determined.
This information can then be processed and used for a wide variety of industrial and military applications.
A problem that often arises in connection with laser illuminated apparatus is known as "speckle." If the characteristic dimension of the surface roughness of the object upon which the laser impinges is on the order of the laser light wavelength, a speckled pattern of relatively dark and bright spots arises. The speckle pattern is due to the interaction of constructive interference among the components of light that produces the bright spots, and destructive interference that produces the dark spots. In particular, the degree of speckle and the locations of the speckle is directly related to the angle of incidence between the illuminating light beam and the surface. The surface roughness contributes to the angle of incidence.
Speckles create problems, particularly for the machine vision apparatus described above. For that apparatus, it is important to be able to determine the pixel upon which the reflection, from the surface of the object to be imaged, of the interface between a light plane and the darkness adjacent the light plane, falls. If the portion of the reflection that falls on any given pixel is a speckle (either a dark or a light one), it is difficult to determine whether that location is part of the light plane or the darkness.
Methods are known for reducing the effect of speckle, but they are not suitable for use with a projected laser pattern. For instance, some methods require defocusing the optics or opening the aperture. For the apparatus described above, it is important to have narrowly focused strips over a large depth of field, which precludes the use of these methods.
One method for speckle reduction, used in connection with a different type of apparatus, is applied to diffused laser light to produce a uniform area of illumination. The light is initially diffused. The diffused light is reflected from a mirror that is vibrating, so that the reflected, diffused laser beam is projected to form a pattern. This pattern is projected through a second diffuser (not the ultimate surface to be illuminated) and is again diffused. It is then collimated onto an image plane. The patent asserts that the speckling is eliminated, because some degree of random spatial phase modulation is produced.
This method of eliminating speckle would not work with the machine vision apparatus described above, because either of the diffusion elements would destroy the initially projected pattern, which is required to be projected on the item to be imaged for the machine vision apparatus to operate.
Another known method of minimizing speckle in a photolithographic illumination application entails irradiating a single laser pulse onto an object and using a scanning mirror and a fly-eye lens to create a uniform, coherent illumination. The speckle pattern produced is moved in at least one direction. The object of this device is to generate a uniform pattern illumination field. This method cannot be used with the machine vision apparatus, because it requires "structured light" which has identifiable geometric pattern throughout the scene. Uniform illumination does not provide such a pattern.